Various types of light diffusion sheet/films used for backlight optical systems of liquid crystal televisions (hereinafter, referred to as “TV(s)”) are provided for the purpose of diffusing light from a light source as uniformly as possible at a designed luminous intensity distribution characteristic, so that an image of a lamp used as the light source is not directly viewed when such a liquid crystal TV is viewed. In order to increase the light utilization factor, a light diffusion film having a high total light transmittance is desired. As a value representing the degree of diffusion of light, “haze value” is widely recognized and used. In order to discuss the degree of diffusion of light more fundamentally, the concept of “luminous intensity distribution characteristic” is used. The “luminous intensity distribution characteristic” refers to the magnitude of luminous intensity in accordance with the output angle of light with respect to an optical film. In general, the luminous intensity distribution characteristic may be different even when the haze value is the same. Conversely, once the luminous intensity distribution characteristic is determined, the haze value is uniquely determined. The luminous intensity distribution characteristic of a diffusion sheet/film is an important characteristic which significantly influences the viewing angle of a display.
Various types of existing diffusion sheets/films include, for example, a diffusion sheet/film produced by fixing acrylic or glass bead filler on a film substrate of polyethylene terephthalate (hereinafter, referred to as “PET”) with a binder resin, a diffusion sheet/film produced by diffusing diffusion fillers in a resin film substrate, a diffusion sheet/film produced by embossing a surface of a resin film, and the like. From these types of diffusion sheets/films, an appropriate diffusion sheet/film suitable for a desired optical characteristic is selected.
A conventional light diffusion film produced and sold by the applicant of the present application (trade name: FE-2000M01; hereinafter, referred to as the “conventional product”) is widely used for backlight units of liquid crystal TVs. This conventional product is a diffusion film produced by embossing a surface of a resin film described above, and the resin film is formed of polycarbonate (hereinafter, referred to as “PC”). This conventional product is used as follows most widely. A surface of another optical film having no useful optical characteristics and a rear surface of this conventional product are bonded together with a pressure sensitive adhesive film.
Recently, the system for mass-producing high luminance white light emitting diodes (hereinafter, referred to as “LEDs”) has been almost established, and the cost of LEDs themselves has been lowered. For these reasons, TV manufacturers have started using LEDs instead of conventional cold cathode fluorescent lamps (hereinafter, referred to as “CCFLs”). From a comprehensive point of view, use of LEDs instead of CCFLs reduces the power consumption of liquid crystal TVs. Use of LEDs instead of CCFLs also influences the quality of light as well as the power consumption. CCFLs, which emit light by illuminating a fluorescent material attached to the entire inner surface of a cylindrical glass tube with ultraviolet, act as so-called surface light sources (assembly of an infinite number of point light sources). By contrast, LEDs act as point light sources.